text:
Manifold pressure vs power vs rpm
From what I know MAP is a way of measuring engine power with variable pitch propeller. Greater throttle setting - greater power - greater MAP on the gauge. It happens because MAP is in fact vacum pressure so opening the throttle gives greater fuel and air flow into the manifold, explained here:
Why does manifold pressure increase with power?
But I also read somwehere that when we reduce rpm without touching the throttle, the MAP will increase, according to this topic:What happens to manifold pressure reading following a reduction in both throttle and RPM?
Reducing throttle decreases manifold pressure. Reducing rpm reduces suction, so the MAP should then increase?
The question is:  why does reducing rpm increase MAP?

I really don't understand your question @Konrad.

You need to actually ask a question.  You have simply referenced two other questions with answers, but haven't sufficiently described your confusion with them.

@Konrad. Sorry they closed this question.  If it helps, try to imagine the (intake) manifold as a room between 2 doors with a giant fan pulling air through door 2.  The fan is your pistons trying to pull air into their cylinders.  If you partially close door 1 by reducing throttle while the fan is running at the same speed, the pressure in the room drops. If you slow your fan down (reduce RPM) the pressure in the room goes up.

Thanks, that is very good explanation. So, on the other hand if we do not touch throttle and increase rpm we will see a drop in map and decrease in power?

Try this Flying Magazine reference, it may help.

Thanks, this article is great and explain everything.

@Konrad, Power is only proportional to MP at constant RPM. At higher RPM the same MP corresponds to higher power.

@Jan Hudec that may be the source of the confusion, because if the airspeed is high enough, reducing pitch may increase rpm by "windmilling" the prop, even without adding throttle.  Increased rpm does not necessarily mean more throttle.  One is power, the other is load.

@RobertDiGiovanni, we are talking about constant-speed propellers, so increasing RPM means advancing the prop lever, which commands the governor to find a pitch for the higher RPM. If the throttle stays put, the MP will drop as the engine is trying to pull more volume through the same opening. I am not sure what power will do though, because the throttle drag increases and the propeller drag increases, but the engine will still be pulling in and burning a bit more fuel. I am guessing it should increase just a little bit.

A little bit here on how the "governor" helps out.

I've got few more questions: if we move prop lever we're changine rpm and then pitch is changed or by moving prop lever we change blade angle and then rpm adjust to it. And  on the tachometer of airplane with variable prop we have engine rpm or prop rpm, because I heard lot of versions.

And one more, according to the answer that I pasted below (I found it somewhere it the internet) everything there seems to be logic BUT if we maintain power setting and decrease rpm the pitch angle should increase. So constant-speed prop shouldn’t generate same amount of power because of blade angle change? I understand that growth on MAP gauge reading while decreasing rpm but I don’t got why power is decreasing, we have constant speed prop, so shouldn’t it reamain constant while changing rpm?

Assume cruising at 4,000 ft, throttled to about 20 inches MP and 2,000 rpm. Now reducing the rpm to 1,200 without changing anything else will cause the MP to rise sharply. The ambient pressure hasn’t changed, the throttle plate hasn’t changed. The only thing that has changed is the speed at which the pistons are pumping the air. Since they are moving much more slowly at the lower rpm, they are not sucking nearly as hard – not creating as much of a vacuum – so the MP goes up, towards ambient pressure. The pistons are sucking far less air, the speed of the air going through the intake is less and fuel flow is less. This means there is less power being developed, in spite of a much higher MP.


text:
Does induced drag drop when wing is stalled?
From formula, induced drag is function of lift. When wing stalls, lift is reduced so induced drag must  also be reduced?
Stall decrease static pressure at suction side of wing, so I know pressure drag must increase, how then induced drag decrease if this is also pressure drag or how can I distinguish these two drag?
$D_i = \frac{L^2}{\frac{1}{2}\rho V^2   \pi b^2 \epsilon}$

@sophit I added in text

When wing stalls, lift is reduced so induced drag must also be reduced?

Yes. But the form drag increases more so the overall drag increases. And it's not like you could actually distinguish them anyway.

Stall decrease static pressure at suction side of wing, so I know pressure drag must increase

Does it?
Well, “suction side” is a bit problematic here, because lift and pressure drag depend on the pressure distribution differently. Lift is produced by low pressure on the top, and that basically ceases in stall, but pressure drag is produced by imperfect pressure recovery around the trailing edge, and that gets worse in stall.

how then induced drag decrease if this is also pressure drag

Induced drag is pressure drag, but not all pressure drag is induced drag.

or how can I distingusih these two drag?

Well, you can't really.
The formula for induced drag, with $\epsilon = 1$, is the minimum drag a passive aerodynamic device must produce to generate lift. Producing more lift violates conservation of momentum and energy—producing upward force, lift, requires accelerating air downward by principle of action and reaction, but then it's kinetic energy increases and if the device does not have additional power input (passive), it comes at expense of the vehicle as drag.
In the non-stalled regime, this formula with added efficiency factor $\epsilon$ plus another term proportional simply to dynamic pressure works well to approximate the drag.
But in the stalled regime the drag is non-linear. You can't use simple formulas to approximate it. So nobody bothers, and therefore nobody cares about which portion of drag is induced and which is parasite—when you do full fluid dynamics calculation, there is simply a pressure field and integrating it results in a force that has a lift and drag component, but the drag is not split to induced and parasite; it just is.

If I integrate static pressure around the wing at 14° AoA, this reslut will be the same as I use simple formula in my post(for non-stalled case)?

@user207141: For non-stalled, subsonic case the simplified formula for total drag, that is $C_{d0}q + \frac{L^2}{q\pi b^2\epsilon}$ ($q$ = $½\rho v^2$ to keep things simpler), will be close enough to what you get by integrating the pressure around the wing for practical purposes.

first part is pressure drag for zero lift , second part is induced drag, where in second part is pressure drag, why there is no pressure drag for case where wing produce lift? Why formula show that pressure drag is not changing with AoA, and where is here friction contribution?

@user207141 there is pressure drag when the wings produce no lift, it's major component of the parasite drag term. But yes, the parasite drag term includes also friction drag; my understanding is that in the subsonic regime they are both proportional to the dynamic pressure, so they can be easily approximated by one common term. As you approach transsonic, things get more complicated—and I haven't actually seen much in a way of simple approximation formulas there.

Pressure drag at zero lift and at 14° AoA can not be the same, so I dont understand why they neglect pressure drag  at non zero lift case...

@user207141 there's a bit of confusion: the first term is not pressure drag rather "parasite drag", while the second term is "drag due to lift". Parasite drag mainly comes from skin friction forces while drag due to lift mainly comes from pressure forces.

@sophit Why do you think formula for induced drag will get  same result as I integrate static pressure around the wing at14AoA?

@user207141: I don't get the question, sorry...

Well, considering the equation you posted if some part of the wing stalls then:

lift $L$ diminishes;
the spanwise lift distribution gets worse in respect to the optimal distribution it had before stall and this reduces the Oswald efficiency factor $\epsilon$.

Whichever of these two effects is predominant determines if $D_i$ increases or not.

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What are the effects of placing the vertical stabilizer at the front of the aircraft?
I am experimenting with different aircraft designs. The one I am trying right now is a canard (horizontal stabilizer at the front) and the main wings are quite far back. I wondered if it would be of any use to put the vertical stabilizers at the tip of each wing as a winglet, but that would not have much direction control effect as the wings are rectangular (not swept back).
The other and best option for me would be to put the vertical stabilizers at the tip of the horizontal stabilizers as a winglet, as shown in the picture.
Problem is, as I said before, the horizontal stabilizers are at the front and this then means the vertical stabilizers would be at the front as well.
Are there any disadvantages to putting them at the front? Would it even work properly?
Thanks in advance and have a great day
Unusual Edit:
As a canard design could sweeping the wings back a little add enough yaw stability to make small forward flying rudders feasible? (Pterodactyl inspired).

It is supposed to be an electric aircraft. I want the cockpit to be at the front and the batteries at the back. Batteries are really heavy, that is why I put the main wings at the back to support the weight. Because of that, I had to put the horizontal stabilizers at the front

Turn it around and you have a Lockheed Electra.  Wing fairings look great!  There is no reason at all why the battery and cockpit CG cannot be forward.  You then have a fairly classical (and much safer) design.  Just put 2 engines on the wings.

@RobertDiGiovanni It is also a matter of space in the aircraft. The whole thing including the fuselage is made of airfoils, that means I do not have much space at the front. I have made calculations, and if the results are right, I cannot really place any batteries inside the actual cockpit, or there will not be enough headroom (for example). I could swap the placement (putting the cockpit at the back and batteries at the front), but the visibility would then be worse.

We have not one but two questions here which cover the subject adequately. I vote to close it.

@Vincent Cerowski Any fuselage "airfoil" will not be as efficient as a wing (but will be very stylish looking).  The wing fairings are good, but let the wing do the lifting.  A fuse can be very pedestrian as long as it does its job.  It need only have a low Coefficient of Drag.

You can double your fuse volume by adding a "bottom half".  See albatross design.  Amazingly, both Pterodactyl and Albatross are ocean hunters, who would benefit from an unstable, computer controlled configuration (to catch fish).  With a stable set up aft, anything that creates a turn must do so against the stable tendency.  A smaller, lighter moving rudder forward (the head) is possible and offers a weight savings and greater maneuverability.  Great for a small bird (who can also manipulate its wings and tail), but not desirable for a larger aircraft.  Birds are better at flying.

But we're working on it.

@mins: Canards must be controlled by computers? I wonder if anyone told Burt Rutan that?

@jamesqf: Canards have been used since the first years of aviation without computers, by Wright and Santos-Dumont. Rutan used canards for lift (e.g on EZ). But for pitch control of a statically unstable aircraft, which is the context of the question, manual control of canards is not sufficient, or I miss something.

@mins: But where does the OP say the design is pitch unstable?  Yaw, certainly, as a consequence of having the vertical stabilizers in front.

Putting vertical surfaces at the front is pretty much trying to make the plane fly backwards in the yaw axis.  Remember that it is just a weathervane. It will want to switch ends same as a weathervane would if you reverse it to the wind direction and let it go.  If you tried to fly a plane configured like that, you'd crash as soon as you got airborne, unless you had some fancy FBW electronics and servos actively controlling the surfaces to force artificial stability in the extremely unstable configuration, and what's the point of that?
The vertical neutral point, the aerodynamic center of yaw forces, has to be somewhere aft of the CG for the weathervane effect to work.  So the fin and rudder needs to be at the back to place the vertical neutral point where it needs to be.
The closer to the CG, the larger the surface needs to be, so you can put them at the end of the existing fuselage if you make the fin and rudder big enough.  Jim Marske's flying wings do that; fin and rudder at the flying wing's trailing edge, made very large because the moment arm is so short.
Or you can sweep the wings and place the fins/rudders at the tips of the swept wings, as Rutan did with the "Eze" airplanes.  This provides the smallest surfaces due to the generous moment arms, and is aerodynamically most efficient.
Or you can add a little stinger sort of tail extension to place the fin/rudder farther aft.  That's what Rutan did with the Solitaire motorglider, where sweeping the long wings wasn't really feasible.

I have thought of doing it exactly like the "Eze", but I am not sure if the benefits of placing the stabilizer there outweigh the cons of having swept-back wings (stalling at the tip and other worse low speed characteristics).

"If you tried to fly a plane configured like that, you'd crash as soon as you got airborne", not necessarily, Rutan did try this front rudder configuration. It was not so good, but it was not stupid either. However the rudder was more like a keel, below the wing not to cause lift loss.

But not a front stabilizer configuration; he just put the rudder surface there.  The yaw neutral point still has to be aft of the CG, so large tip fins are required, larger than otherwise, to compensate for the destabilizing effect of the forward rudder, and you're back where you started.  If you sawed off the wing tip stabilizers, it would immediately crash.

If you want to keep the wings straight, then you'd go the Solitaire or Marske route.  I'd probably go the Solitaire route and try a carbon boom to move the fin a few feet back, to the extent you can deal with the tail clearance issues.  You could also try a large fixed fin aft and moveable rudder forward as @mins mentioned, and deal with some probably odd characteristics (like dynamically unstable reactions to rudder inputs since a fixed yaw input results in ever increasing rudder AOA as it yaws).

John, I agree, there is a difference between horizontal and vertical front control surfaces due to lack of wing equivalent in the vertical plane.

Would it even work properly?

Depends on your definition of "properly".
Would it steer the aircraft like a tail one would? In principle yes.
Would it be as easy to use as a tail one? Not by a long shot.
Putting the vertical fin in the front will not make it act as a stabilizer any longer: any small deviation in sideslip angle will get amplified by said surface (instead of corrected). And the reason is that now the surface is ahead of the CoG, instead of behind.

Well, I think the only feasible option now is to put it in the center at the very back of the aircraft and move the main wings a bit forward (to increase its distance from the CoG - I will make it later so that the CoG is where the wings are, as it is now, by moving the inside components - should have some margin). Thanks for the help!

@VincentCerowski Calculate your CG properly. Use https://www.ecalc.ch/cgcalc.php. The size and position of horizontal stabilizers will change the correct CG position. The usual advice of 1/3 or 1/4 wing chord only applies to normal planform with "regular" tail sizes. The more you deviate from it the more that advice is wrong. You are building a canard - something VERY different from "normal" planes.

It's like driving a car in reverse .. doable but much harder !!

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text:
Can lift be created without air downwash?
I was looking at the great site code7700.com, but I don't really understand their explanation of lift and drag:

   The so-called infinite wing
displaces air for a while, but the air particles return to their
original positions along the relative wind. The relative wind
experience up wash as it approaches the airfoil and down wash as it
leaves the airfoil. But the up wash and down wash are equal so the net
effect is cancelled. Because of this, the aerodynamic force occurs
perpendicular to the relative wind and their is no induced drag. In
other words, all of the aerodynamic force is producing lift, none of
it is pulling the wing backwards.
Source

It seems this is a big confusion. Could someone just confirm that lift is created solely by the downwash and Newton's 3rd law of motion?
(Why air is deflected is not part of the question.)

In the world of physics everything "just works that way" and anything we call "laws" are just models. Like the earth goes around the sun once a year not because the law of gravity "causing" it that way, but describe it well. Even now I don't think our understanding of physics knows for sure what "causes" gravity. Same thing here. Most people find the pressure-lift model more useful than the downwash-lift model to study an airfoil. Downwahs-lift model works better for a tunneled device like duct fan or jet where you can measure flow mass and flow velocity accurately not pressure distribution.

is the car acceleration created by pushing the road backwards? or by the engine? or maybe by the torque? it all depends on how you want to understand the question

@user3528438 (and szulat) When you feel the recoil of a gun, you can't tell for sure if the recoil is due to the reaction to the bullet acceleration, according to the 3rd law? Is that what you suggest?

Of course you can model it that way, or you can model it as the chamber pressure pushes the back of the chamber backwards. The choice of the model depends on the purpose. If you study the projectile's ballistics, then pressure model (2nd law) is more convenient. If you study the gun's recoil then the 3rd law's model is more convenient. Sure you can do it the other way but why?

Although they don't directly explain why code7700 is right or wrong here, Dan Hulme and others have good explanations on lift, Newton's law, and oversimplications like this at this question, just in case anyone hasn't read it: https://aviation.stackexchange.com/questions/16193/how-do-wings-generate-lift

@user3528438: I understand what you mean, right of course.

A long-running discussion of some of these ideas has been taking place here-- https://www.rcgroups.com/forums/showthread.php?3148317-Should-NASA-be-teaching-incorrect-version-of-lift-to-our-kids

@quietflyer: Maybe a better reference to compare how pressure, circulation and acceleration can explain lift.

If no downward acceleration occurs, there can be no lift.
However, a wing of infinite span will use an infinite amount of air for lift production, so an infinitely small amount of downward acceleration already suffices to produce lift.
Clearly, this is a thought experiment that cannot realistically be reproduced in reality.

Edit five years later: Essentially, this is d'Alambert's paradox of lift. Force without counterforce. This is only possible in 2D flow, as soon as reality intrudes it will disappear.

you have to use lift per length or down wash per length for all calculations or everything goes to infinity

@user3528438: Since the downwash intensity is inversely proportional to the square of wingspan, this does not really help.

@PeterKämpf  "However, a wing of infinite span will use an infinite amount of air for lift production, so an infinitely small amount of downward acceleration already suffices to produce lift." A lift of infinite span will have aerodynamic properties in each section exactly equivalent to an airfoil. So, you don't have a downwash at trailing edge because it's of infinite length? That's wrong. You have a downwash at the trailing edge but it's because of the airfoil's circulation and is not the combined downwash of 3D wing induced flow plus airfoil local flow.

@AlexandrosK.: No, you misunderstood the question. If you have measurable downwash with infinite span, lift will also be infinite. Here the question is about any lift at all without any downwash. And that cannot happen - an infinitely small amount is needed even for very little lift.

@PeterKämpf Now you're playing with the equation relating wing induced downwash with aspect ratio $AR$ and lift coefficient $C_L$ when I was referring to the cambering of the flow behind the airfoil which is due to the circulation $\Gamma$ (only 2D flow). Check the second figure in my answer below.

@PeterKämpf So downwash in momentum theory at airfoil is infinitely small but not zero?  Does in potential flow theory also predict this infinitely small downwash?

To complete some of the other explanations here, lift is produced (at least the most part, there are other mechanisms by which lift is produced in real aircraft, such as engine mount angle) by downwash. Most if not all phenomenons producing lift can by explained by or can be linked to downwash.
By deflecting air downward, even if the velocity field at infinity is strictly unaffected by the immediate downwash of the wing, it still creates a slight deviation of the airflow close to the wing. This is translated to a pressure field, which combined with Bernouilli's theorem, gives rise to lift.
Basically, pressure lift can be explained by downwash and vice-versa.

Well, the inclined, lift-producing propeller or jet engine also accelerates air downwards. No need to make an exception - all cases of dynamic lift require downwash to work.

@PeterKämpf Absolutely true. Just didn't cross my mind since we were talking about airfoil downwash, but it's basically the same. Thanks.

Sorry but this source is pretty bad. The explanation is clearly wrong. You should download a basics aerodynamic book...
Here's what happen in a finite (real world) wing, the lifting of the wing generates a pressure field which induces vortices running from the lower surface to the upper, these vortices generate the downwash which is explicitly related to induced drag by a simple equation. The strength of the downwash is mainly depended on the lift coefficient and the aspect ratio of the wing, and in the limiting case of infinite wing (which is a theoretical aerodynamic's term) the wing's induced downwash doesn't exist and induced drag is zero.
So, downwash or upwash would depend on the wing geometry and lift, it's lift tha generates the upwash/downwash and also affected by it since it modifies the local angle of attack on each wing's section.
If you're considering the full airplane, the upwash/downwash of wing is also affected by the fuselage near the root.
This picture you posted is very wrong and confusing, because even in 2D flow, you have a cambering of the flow behind the airfoil (of course flow isn't just going flat after trailing edge). The flow then re-alignes with the freestream direction.

This picture. taken by a Stanford's Aerodynamic's lecture course, perfectly answers your question
The Kutta-Jukowski theorem for lift states that lift is proportional to density, freestream velocity and circulation $\Gamma$ by the equation $L=\rho V_{\infty} \Gamma$. For a 2D airfoil the circulation is as depicted in the far upper sketch and will produce an upwash in leading edge region and a downwash in the trailing edge region, then in the second sketch you see the downwash induced by the wing's free vortices. Lastly, you see the combined induced flowfield.

I explicitly mentioned "why air is deflected is not part of the question" to prevent entering useless debates around the cause of the downwash, I'm interested in the consequence of it as lift. So unless you rewrite it, your post is not an answer.

@mins Deflected air changes local angle of attack and hence lift, and the change of lift changes the deflected air, this is strongly coupled.

@mins There is no reason for debates, these are the physical phenomena and are well understood in aerodynamics for the macroscale we are discussing, we're not talking about turbulent, unsteady flow in a post-stall condition where things are more complicated.

There are few points pretty unclear (and that seems false in fact). First, there can be lift without downwash-producing wingtip vortices. A 3D wall-to-wall wing (i.e. 3D airfoil) still produces lift, without wingtip vortices. Kutta-Jukowski's circulation is simply be negative on the underside, and positive and bigger on the upper side, which integrated over the airfoil curve gives a non-zero lift (circulation is greater over than under).
Basically, lift is created by air downwash.

@Cedric 1. What is unclear? 2. Who said there can't be lift without wing induced vortices? Of-course there is, e.g. a 2D lifting airfoil. 3. By saying circulation $\Gamma$ we mean the net effect i.e. the closed-curve integral, you speak about vorticity.

@AlexandrosK. I didn't say you said it. I said it is unclear, because given your explanations, one can assume lift is created by vortices and given the image you reference, wingtip vortices are the most apparent thing on it. Moreover, the second referenced image about circulation do tend blend in with your "upwash" explanations. Which is why I wanted to clear this up.

@AlexandrosK. Basically, I think it is pretty confusing to talk about vortex physics when explaining basic lift-producing mechanics. Those explain pretty well most aerodynamics effects sure, but for the uninitiated, this is all pretty confusing, which is why I wanted to clear this up.

@Cedric Ok, I hope the second sketch made it clear, by separating 2D and 3D upwash/downwash. Again, just to know, it's not only the wingtip vortices, as you see from figure 1 there is a whole vortex system on the wake. Which is why winglets can suppress only locally the effect of vortices.

Downwash is in no way a cause of lift.
Newtons third law (equal and opposite reaction) is a consequence not a cause of Lift. It is a consequence of the principle of conservation of momentum, which must be due to required spatial-translation symmetry.
According to Noether's Theorem, every physical symmetry leads to a conservation law. For example, time-translation symmetry (the laws of physics don't change over time) implies conservation of energy, and space-translation symmetry (the laws of physics are the same everywhere) implies conservation of momentum. But it is Newtons second law (F=dP/dt, where P is Momentum (mass x velocity)) that is most directly associated (but not causal to) with aerodynamic lift.
Lift is a FORCE. Forces can and do exist even in the absence of motion, (see below). If lift can be said to be caused by anything, it is caused by thr exchange in momentum (dP/dt), that occurs when molecules bounce off  the surface of an airframe.
A simple thought experiment demonstrates that downwash is not necessary to have Lift. Inflate an expandable compartment under a weight with gas (or air) under pressure. What is holding up the weight? Clearly there must be a LIFT force! Just as clearly, there is no downwash as there is no no fluid movement, (except Brownian motion).  To see a more detailed and academic explanation of this, (and other aerodynamics misconceptions), review this you tube video

+1. You're right to remind action-reaction is a case of momentum conservation, and therefore not an initial cause. Downwash is nevertheless a convenient intermediate result to compute lift without restarting to Navier Stokes equations. If I'm not wrong this is actually the idea behind Kutta Condition which by itself is an answer to my initial question: "Can lift be created without air downwash?"

@mins and CharlesBretana: Newton's third law has nothing to do with energy.   Newton's first, second and third laws translate in simple words the conservation of momentum.   There's no causes or consequences involved, just sides of the same coin.   Molecules do not bounce off of the surface of an airframe unless the airframe is flying hypersonic.

@Sophit, yes my bad, you are correct about 3rd law. But Simple thought experiment that demonstrates that downwash is not necessary to have Lift. Inflate an expandable compartment under a weight with gas (or air) under pressure. What is holding up the weight? Clearly there must be LIFT!  Just as clearly, there is no downwash.

@sophit, thanks did as you suggested.

"Inflate an expandable compartment under a weight with gas (or air) under pressure [...]" if I understand correctly this looks like a balloon. There is no lift in such aircraft, only buoyancy.

Define "Lift". If you are excluding the force generated by the  differential between the pressure on the bottom and the pressure on the top of an object, unless it is not moving, then your definition is a circular one.

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text:
Efficiency of Coaxial Rotors vs Single Rotors (and Contrarotating Propellers)
Why are coaxial rotors (or contra-rotating propellers) used given that there is such a high power efficiency penalty?
According to data I see in Principles of Helicopter Aerodynamics by Leishman,it seems that coaxial rotors have lower figures of merit than single rotors. According to the theory it can take up to 28% more power for a coaxial rotor to provide the same thrust as a single rotor in hover. The book also shows that even partial overlaps are less efficient than single rotors.
However it seems like there is this perception that coaxial (or overlapped rotors) are more efficient than single rotors. Some helicopters (Kamov series, K-Max), several multicopters and eVTOLs use coaxial/overlapping rotors. What gives? It seems like an excessive power penalty (+28%!) to take given in theory most designers are attempting to minimize power consumption for a given performance setting.
I was trying to understand this, and the only explanations I could devise were:

In theory the Figure of Merit of a Single Rotor doesn't consider the power to provide anti-torque. This is quoted as being 10-30% depending on where you read.

Some geometric or other constraint in the design forces the use of coaxial rotors. EX: desire to have no tail rotor incidents, or need to limit rotor diameter or tip speed.

Similarly, I notice the same trend on contra-rotating propellers. But I find the data I have is not very trusted (random online papers). Does anyone have good references to compare efficiencies of single propellers vs contra-rotating propellers?
Thank you!
Added reference picture!

The premise of your question is not correct. Presumably 28% is the difference between rotors performance only, at equal disk loading (it's lower for coaxials), and without accounting for the tail rotor, which is a pure waste of power.
This has been researched:

The analysis of hover, forward flight, and steady level turns shows
the benefit of the coaxial rotor to lie in its reduced consumption of
induced power compared to the equivalent conventional rotor.
...
Simulations of a wind-up turn at constant advance ratio again show
the coaxial rotor to possess a distinct advantage over the conventional
system — a reduction in power of about 8% for load factors between 1.0
and 1.7 is observed at an advance ratio of 0.12 and a level flight thrust
coefficient of 0.0048.

Also, on a practical level, one can compare the Mi-28 with the Ka-50:
https://en.wikipedia.org/wiki/Mil_Mi-28
https://en.wikipedia.org/wiki/Kamov_Ka-50#Specifications_(Ka-50)
They use the same powerplant and are similar in size, weight, and fuel capacity.
Despite its 14.5 m vs 17.2 m rotor diameter, the Ka-50 has a longer range, confirming the theoretical results - coaxial rotor helicopters are more power- and fuel-efficient.

Comparing the ranges of the Mi-28 vs. the Ka-50 is unfair since they have two very different powerplants and, most probably, fuel capacity...

@sophit They have a similar fuel capacity of ~2000L for either, and they use the same Klimov VK-2500 powerplant.

Yes but according to Wikipedia one is rated at 2400shp and the other at 2200shp. Plus they have different fuselage and shape i.e. different drag... a comparison is simply unfair. To do a comparison you should have exactly the same helicopter, one with coaxial rotors and one with conventional rotors or at least a bunch of machines to be able to make some sort of extrapolation

@sophit Or, better, remove the whole helicopter and work only with the rotors - as has been done in the research paper in my first link. The comparison is only to confirm that a real coaxial exists that isn't worse on fuel efficiency than a conventional.

Thank you for sharing that report! Took me a few days to read it but I think I understand. I suspect you're right that the graph from Leishman only shows the rotor effects and does not consider things like power consumption for tail rotor operation, or rotating wake vs airframe effects.

I suspect that you are not looking at the Leishman data in an entirely consistent way.  For example, you may need to compare the contra-rotating rotor to a single-rotating rotor with 2x the disk loading.  Can you cite the exact figure/page you are looking at?
Theory says that contra-rotating coaxial props / rotors should be able to be designed to be more efficient than single-rotation counterparts of equivalent disk loading (considering total thrust and total diameter).
This improvement is mostly based on designing the second rotor to turn the swirl component of velocity induced by the first rotor back 'straight' -- leading to a combined system with no swirl velocity.  The kinetic energy normally lost in the swirl component is now recovered in the axial component.
Gas turbines are designed in a similar way with rotor/stator pairs.
Many eVTOL concepts using counter-rotating rotors are doing so for a variety of reasons -- they want to keep the simplicity of a quad-rotor configuration, but they want to get redundancy of totally separate motor/drive/rotor systems at each post.  This also allows them to use smaller components (motors and drives) compared to a true quad-rotor system (for the same gross weight and diameters).  This may be beneficial when considering what components are available off the shelf.

Hi Rob! Thanks for your answer! I added the picture of Figure 2.30 that gave me the inspiration for this question!

How would I compare a single rotor to a coaxial rotor correctly using the the chart?

note that the swirl can help prevent the recirculation bubble from starting when descending at low forward speed.

I've gone through and read that section of Leishman.  It is a bit confusing exactly how he is comparing in each situation -- and how you would use this chart.  However, he stands clear that the coaxial is less efficient in the low disk loading situations usually used for rotorcraft.  He does draw explicit contrast to high disk loading propellers where we usually learn that counter rotating props are usually more efficient.


text:
How do a helicopter's cyclic and collective transfer commands to the swash plate?
I understand the function of the cyclic and collective, and I am specifically wondering if anyone has a good image/video showing the mechanism of how the flight control "mixer" works! I can't seem to find many good illustrations of the mechanisms themselves, even a simple example.
Perhaps something like this but clearer, or something filmed in real life?

Edit: I circled the area in question in the picture below. Basically I understand how the cyclic and collective sticks move, and how the swash plate moves, but how do the stick movements move the swash plate actuators correctly?
I think I generally understand what is going on, and I understand what the swash plate is supposed to do, and which direction the swashplate actuators are supposed to move for a given maneuver. But what does this look like in real life? Is it typical to have just two actuators to the swash plate?
And in this system it seems like adjusting the collective would make the pilot's control column move? Is it common?

For example, I can understand in this picture that the cyclic stick can move forward and aft (circled in blue), which moves a control rod around a pivot, which lifts the forward actuator on the swash plate. I guess I am mostly confused as to how the collective plays in, and would like other diagrams or images to understand.

This is an image of a mirrored cyclic linkage as I understood it from @Pilothead:

Thanks!

Are you talking about the linkage below the rotor in the middle?  Can you draw a circle on the area you are talking about on the posted image?

Hey John K, I've added some edits and circled some parts of the drawing! Basically I understand how the cyclic and collective sticks move, and how the swash plate moves, but how do the stick movements move the swash plate actuators correctly?

The mixer is mainly found in larger helicopters where their expense is not such a large part of the purchase price. This answer explains more of the function of the flight control mixer: it de-couples degrees of freedom from the control inputs.
The helicopter in OP picture shows unmixed inputs into the swash plate: tilt swashplate forward for longitudinal control, sideways for lateral control, pedals connected to tail rotor only. Picture below is from Principles of Helicopter Aerodynamics by Leishman, and shows a schematic of the cyclic longitudinal
Three actuators are connected to the Non-Rotating Plate, which can be tilted and lifted. The Rotating Plate follows the fixed plate, and cyclically pitches the rotor blades so that they adjust their circular path up/down. The swash plate arrangement is quite complicated, as shown in the photo below, from the same book.

The mechanical linkage from the cyclic and collective sticks to the main rotor actuators does have some input summation involved, as the picture below shows (from this source, which indeed calls it a mixer).

There are always three swashplate actuators. The collective stick always deflects all three simultaneously so that the swashplate lifts, while cyclic stick movement deflects one or two in such a way that the swashplate tilts.

Thanks Koyovis! I guess I had not realized that the term "mixer" specifically referred to a system where the swash plate is not mechanically connected to the flight control stick. I should clarify my question to reflect that I am trying to understand how an unmixed helicopter swashplate receives commands from the control sticks.

@Gerry Mixers can be mechanical as shown in your diagram. All this means is that multiple control inputs, like cyclic and collective, are 'mixed' together to get a different set of outputs, like swashplate height and angle. The horizontal bars in your large blue oval are the mixers, actuated on one end by the cyclic and the other by the collective.

Makes sense! So if the pilot uses the collective, will he/she feel a change in pitch in the cyclic stick that he would need to correct? I do not fly helicopters so I don't know if that is normally the case.

There is some interaction between the flight controls, not very seriously. Particularly noticeable in large helicopters with long linkages from stick to swashplate - when collective is pulled, some feedback force can be felt in the cyclic, easy to overrule. Never the other way around.

Well that was a revelation to me! I had no idea that the controls were not completely "decoupled" and was really shocked. I suppose that despite that, the helicopter is still controllable. Thanks for clarifying Koyovis!

The required motion of the swashplate in order to achieve a desired motion of the helicopter is not as simple as your illustration supposes. Due to gyroscopic precession, tilting the swashplate forward does not tilt the helicopter forward but to one side. This video explains how. The principle that pitch and roll can be controlled by tilting the swashplate to get differential lift is also explained.
The collective controls climb and descent. This is achieved by increasing pitch on all blades simultaneously, or collectively, explained towards the end of the video.

In your diagram, lifting the collective uses linkage to raise the swashplate without tilting it as shown by the arrows.

Hi Pilothead, thanks for the explanation! So if I understand, the actuators on the static portion of the swash plate are placed 90 degrees from where they should be in the image, correct? Also, as far as I understand if the pilot uses the collective, will he/she feel a change in pitch in the cyclic stick that he would need to correct? Is it a flaw in this diagram, or is it how typical helicopters work?

@Gerry Yes, roughly 90deg off. Use of collective does not affect cyclic, though your simplified diagram looks like it would. It does not show any offsetting force which could be provided in multiple ways, as by a mirrored cyclic linkage.

Would you happen to have a better diagram, or one that shows a mechanism where the collective does not affect the cyclic? By a mirrored cyclic linkage, you mean that for this diagram's system, there is a linkage missing that would push the cyclic down at the stick level if the collective is raised?

I don't have a diagram, but it is simple. Take the longitudinal cyclic linkage right in front of the collective linkage in your diagram, and mirror it to the rear so that it works opposite to the front linkage. When the collective raises the swashplate, the opposing cyclic linkages cancel out any force because they only move opposite to each other.

I think I understood, and I added a drawing of my understanding to my original post. Thanks!

Video worth watching
YOUTUBE - hydraulic system test of Mi-24 rotor hub


text:
How does the Boeing 787 Dreamliner's glide ratio compare to other airliners?
I just saw a documentary on the design of the Boeing 787 "Dreamliner" and one of the designers said that the wings were inspired by gliders. Looking at the production version of the plane, it is obvious that the wings are indeed more glider-like than other airliners.
How does that affect the glide ratio? I can't find any data on this (is there any data on that at all?)

As discussed here, glide ratio is related to the L/D (lift-to-drag) ratio of the plane. When talking about commercial aircraft, "glide ratio" is typically used to mean the ratio with engines inoperative. If the jet engines aren't running, they add a lot of drag, which will decrease the L/D ratio from the typical max value.
This analysis estimates the max L/D of the 787 at 20.8, while this site estimates 21.
Other sources estimate L/D of the A330 and A340 at 20. The 747 is estimated at about 18. So based on these estimates, the 787 is slightly better than other large airliners.
Most of the efficiency gains of the 787 are from the lower weight of composite materials (which allow bigger wings) and more efficient engines.

Efficienciency is geometric, it has nothing to di with weight. Only the speed where you'll find that efficienciency is influences by weight, this is why gliders are often loaded with water.

@Caterpillaraoz is correct that L/D (and therefore glide ratio) is independent of weight.  The final paragraph of the answer (efficiency gains) addresses a separate issue (weight of the airframe).  A lighter airframe requires less lift for level flight, so there is less drag, resulting in lower thrust requirements.

There are many factors to glide ratio of an airplane, but a good indication is the aspect ratio (see more here: Wikipedia ). The aspect ratio tells us how "slim" the wing is.
The slimmer a wing, the less induced drag it will create. The downside is that the longer a wing is, the more parasitic drag it creates. So it is a tradeoff for glider designers. For low speed flight, the longer the wing, the better. But for high speed flight, a short wing is better. This can be seen in open class gliders, where the trend the past 30 years have been to create greater wingspan gliders. The last few years, this has changed, and glider manufacturers are now producing new generations of open class gliders that have shorter wingspans than their predecessors(in the case of the new Quintus vs its predecessor, the Nimbus 4: 23m vs 26,5m)
In gliders, a high aspect ratio in nearly all cases (there are exceptions) translate to a high glide ratio compared to low aspect ratio gliders. Examples of gliders from the same generation (or close to the same) and their aspect ratios and (theoretical) glide ratios:
(AR: aspect ratio; GD: glide ratio)
LS4: AR: 21,7; GR: 40
Ventus 1: AR: 23,7; GR:44
Discus: AR 21,3; GR 42
Nimbus 3: AR 35,9; GR 58
ASH25: AR 39,8; GR 60
Now an extreme example that has been produced in very few numbers; the ETA has an aspect ratio of 51,3 and a glide ratio of 72.
Now, enough about gliders! If we take airliners, then the Gimli Glider (Boeing 767) had a reported glide ratio of 12:1 in an emergency situation.
A 767-200 has an aspect ratio of 8~, along with a glide ratio of (according to Wikipedia) 12:1.
An Airbus A340-300 has an aspect ratio of 10~, and a 737-NG with winglets has an aspect ratio also of 10~. On the other hand, a 787 has an aspect ratio of 11~.
But without factory numbers of any of the airliners, it would be hard to say. But in any case, going by the aspect ratio, the 787 should have a better glide ratio, compared to the other airliners i mentioned.
With regards to the 787 wing features, then the leading edge sweepback is very popular among glider manufacturers today. There is also a noticable increased dihedral(wing bending upwards) further out the wing, this is also seen on gliders.
It is hard to suggest if these factors will increase the glide ratio of the 787 compared to other planes, as the wing is certainly designed for optimal performance at cruise speed, and the speed for best glide ratio is likely alot lower.
The only conclusion i would guess at, is that because of an aspect ratio that is about 10% higher than comparable airliners, then the 787 gliders better (but i have no idea of how much). If i would have to ballpark it, i would guess somewhere around 16-18 in glide-ratio.

The part about aspect ratio is right on but you go off the rails with "conclusion I would guess at"

Total guesswork here, but a typical airline will have a glide ratio of 15-20:1. Gliders  attain 30-60:1.
According to Boeing, the wing has a "highly efficient lift-to-drag ratio", which to me means 20-25:1. So that's my estimate.

Please explain downvote. I answered the question using educated guesswork and sources, and no one else has answered with an official source.

I didn't downvote, but I can understand why it was downvoted. Guesswork is not really a valid answer.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What are the US Customs requirements upon returning to the US after pre-clearing customs in the US Virgin Islands?
I have heard conflicting information regarding pre-clearing customs in the USVI.  On one hand, I have heard that once you clear customs you are done and don't need to do anything special on arrival.  But the I've also heard that you need to land at an Airport of Entry upon arrival and notify local customs of your arrival.  I have even called Customs and Border Protection and gotten conflicting answers!  What is actually required?

You're required to notify and clear customs if you land in the 50 U.S. states or Puerto Rico from the U.S. Virgin Islands, even though the U.S. Virgin Islands is part of the U.S.
"All private aircraft arriving in the United States via the Gulf of Mexico or the Atlantic
Coast from a foreign location in the Western Hemisphere south of 30 degrees north
latitude, or from any place in Mexico or from the U.S. Virgin Islands, shall also furnish a
notice of intended arrival. Such aircraft must then land for inspection at the nearest
designated airport unless an overflight exemption has been granted under Section
122.25 of the Customs Regulations."

Where is this quote from?  Also, my question is about pre-clearing in the USVI (with US Customs), after which you are considered a domestic flight in most regards, so this doesn't apply.

An FBO in the USVI offers the following explanation:

When an aircraft that is inspected by CBP in the U.S. Virgin Islands arrives in the United States, it is considered to be pre-cleared.

However, you must be able to provide evidence of the inspection, and must arrive at an airport with CBP facilities unless that requirement is waived by the CBP.
19 CFR § 122.24 discusses "Landing requirements for certain aircraft arriving from areas south of U.S.", which states that aircraft must provide advance notice of arrival and stop at the nearest "designated airport", but also includes the following exception:

This requirement shall not apply to those aircraft which have not landed in foreign territory or are arriving directly from Puerto Rico, if the aircraft was inspected by CBP officers in the U.S. Virgin Islands, or otherwise precleared by CBP officers at designated preclearance locations.


text:
How is RVSM approval obtained for a properly equipped airplane?
If an airplane is already equipped for RVSM flight, how do we obtain a Letter of Authorization (LOA) for RVSM in order to actually fly in RVSM airspace?

You can find lots of information on RVSM at this FAA website.
From This MSWord document found on that site:

Operational Approval or Authorization.   The FSDO or CMO will grant operational approval or authorize the operator to conduct RVSM operations by issuing Operations Specifications paragraphs B046 and D092 to air carriers or issuing a Letter of Authorization (LOA) to general aviation operators, as appropriate.  Individual operator RVSM-compliant airframes are listed in D092 or on the LOA, as appropriate.  The FSDO or CMO will take this action after it has determined that the operator’s aircraft are RVSM compliant; it has approved the operator’s RVSM maintenance program; a CMO has approved or a FSDO has accepted the operations programs and the office has accepted the operator’s plan for participation in the monitoring program.

This Excel spreadsheet lists points of contact at various FSDOs.


text:
Under what conditions can a non-RVSM aircraft or pilot fly in RVSM airspace?
After purchasing an aircraft, it takes the FAA a long time to issue RVSM approval for the new owner.
During this time, under what conditions may an aircraft fly in RVSM airspace even though they aren't approved for it?
What is the process to get ATC approval for such flights?

According to the AIM Section 4-6-8, the language to use with ATC is simply, "Negative RVSM."  You have to say it on the request for an RVSM flight level, flight level change, or climb/descent through RVSM airspace.  You must also say "Negative RVSM" on check-in if currently at an RVSM flight level.
NBAA provides a guide for navigating the RVSM approval process.  This guidance is fairly new, and indicates that approval through a prior operator of the airplane is acceptable.  Your FSDO will make the authorization.
See also FAA RVSM Documentation

A non-RVSM aircraft can't fly in RVSM airspace for a final altitude. But they can transition it, e.g. climb to FL430.
There are exceptions to the rule, covered in FAA JO 7110.65 2-1-28(a)1:

military operations
manufacturer aircraft being flown for development/certification
foreign state aircraft
medivac flights

Beyond these, there isn't a way to get approved to fly at an RVSM altitude without the certification.


text:
What are Operations Specifications (OpSpecs)?
I had never really heard of OpSpecs until I started a new flying job that was at a Part 135 company.
So for those people who are new to this (or are just interested):

What is an Operation Specification and how does it relate to the actual CFR regulations?
What kinds of things are they used for?
Who uses OpSpecs?
In general terms, how is an OpSpec obtained?

Putting aside the legal definition for someone else, they are a set of rules that an operator (an airline for example) agrees to operate by. The FAA has to approve them. The ops specs can be more restrictive than applicable FAA regulations, but never less restrictive (as I remember). They cover many things, some of which (that I can remember) are:

Type of equipment you're going to operate.
Where you're going to operate.
Extent to which VFR operations will be allowed.
Types of navigation equipment permitted.
Who you can offer your services to.
Crew training required.
Duty and flight time limitations.
Disciplinary protocols to follow.
Flight planning, dispatch, flight following requirements.
Lists of who is who: chief pilot, flight standards director, etc.

There are a lot more areas, but basically it's how you're going to operate your airline. The thing to remember is to give yourself as much leeway as possible. Don't do something stupid like saying that if someone fails an alcohol or drug test that you're simply doing to fire them. Provide for as much VFR operation as you can possibly get the FAA to agree to. Don't saddle yourself with impossible maintenance requirements if you have a mechanical problem in, say, Harare.
Ops specs are best written by someone who has been around the block many times. Write them poorly and you're going to cost yourself a lot of money that you would otherwise not have to spend. If you get a recalcitrant FAA inspector, it's worth fighting it out. Appeal as necessary until you get what you need. Don't be afraid to show up the ignorance of FAA newbies that may have been assigned. Also, remember that the FAA regions are not standardized. It may even be worth considering changing the region where you're technically based to get what you want.

OpSpecs are pre-written templates these days, and you either get them or you don't (unless you get a deviation).  The last two paragraphs are great advice for an FOM/GOM and other manuals though, and the beginning all applies to OpSpecs!

@Lnafziger When they started requiring OpSpecs for Part 135 operators, pre-written templates quickly appeared. However, I can't imagine a pre-written template being acceptable for a Part 121 or Part 125 carrier. But then I've been retired since 1999 and perhaps things have changed more than I realize.

An OpSpecs is basically a contract between the FAA and the Carrier. It grants permission for the carrier to operate and contains the permissions for several parts of the operation. The OpSpecs are created and agreed upon before the carrier receives there certificate to operate.
The carrier's (certificate holder's) Manual should ensure that the OpSpecs are upheld or the FAA could pull the carriers certificate. See the following (one is for Part 121 but, it explains a lot):
http://fsims.faa.gov/wdocs/8900.1/V03%20Tech%20Admin/Chapter%2018/03_018_003.htm
https://www.faa.gov/about/initiatives/atos/air_carrier/intro_to_part121_cert/


text:
What sources of weather are approved by the U.S. National Weather Service?
Part 135 and (I believe) Part 121 operations all have a requirement to use a source of weather that has been approved by the U.S. National Weather Service:

§135.213   Weather reports and forecasts.
(a) Whenever a person operating an aircraft under this part is
  required to use a weather report or forecast, that person shall use
  that of the U.S. National Weather Service, a source approved by the
  U.S. National Weather Service, or a source approved by the
  Administrator. However, for operations under VFR, the pilot in command
  may, if such a report is not available, use weather information based
  on that pilot's own observations or on those of other persons
  competent to supply appropriate observations.
...

Does the FAA or the U.S. National Weather Service maintain a list of weather providers that have been approved and are legal for us to use?
If not, then how do we know whether a particular source has been approved?

There is an ongoing issue with this, as far as I am aware. First off, this does not apply to part 91, and nothing does as far as I am aware: you can get your weather from anywhere, but having it on record with FSS or DUATs gives you a little more padding if something goes wrong.
If you're going to get weather online, which most large operations do, the FAA requires air carriers to get weather information only from Qualified Internet Communications Providers (QICP). This is outlined under AC 00-62. Or rather, it was outlined there. 00-62 was quietly cancelled in mid 2013. In addition, the only list of QICPs (which would have answered your question) I know/knew about was removed from the FAA website.
A few weeks after it was cancelled I found out about it and called the FSDO to ask (Memphis or Louisville, I don't remember which). The man who should know told me that I knew about as much as he did. Their regional offices hadn't told them anything, and he was telling everyone to continue using the qualified service they were previously using until they learned more.
I have not called them back since, but after a quick search, I only found this OpSpec document and one that it supersedes from a few months before (8900.199). There is also a newer one in the series (8900.242) but it seems to have no information relevant to the question. I must adimit that this document is a bit over my head, but it seems that they did an overhaul of the weather regulations regarding 135 and 121, and that use of a QICP is no longer required. So I take it that unless your carrier is only certified to use a certain system, you're now allowed to use whatever you want to get the weather.
You can read those documents here, the three I mention were published under AFS-200. Search for that and you'll find them and more.

So I did some more research:
Check out 8900.1 Ch 26 Section 2. Regulatory Sources of Aviation Weather Information and Aviation Weather Information Systems – Parts 91K, 121, and 135. The first bit has a good overview of all the weather products and their intended purposes. Relevance to the question starts at 3-2073(A) Weather Reports Prepared by the NWS or a Source Approved by the NWS. (page 6 in the PDF version).
Much further down it says (in reference to part 135, 121, 125, etc certificate holders only):

B. Accessing Weather Information via the Public Internet. Certificate holders and
  program managers often access weather information via the public Internet. When accessing
  weather information this way, certificate holders and program managers are required to use
  weather information that is provided by a weather source authorized by regulation, or approved
  by the Administrator, and is listed in the certificate holders’/program managers’
  OpSpecs/MSpecs. In the past, the FAA maintained a list of approved QICP. However, the FAA
  no longer approves QICPs. The FAA has canceled AC 00-62, Internet Communications of
  Aviation Weather and NOTAMS, and no longer maintains a list of approved QICPs. Certificate
  holders and program managers using the public Internet to access weather information are
  responsible for ensuring accurate and timely delivery of information without data corruption
  during the transmission.

That's all pretty cut and dry, I think, but keep in mind that those OpSpec papers mention that everything in 8900.1 Ch. 26 is under review and has been edited recently and will be further edited in the near future.

This is interesting detail, but would be great to see a list of recommended or most popular sources as well. e.g. 1-800-wxbrief, connecting foreflight to DUATs, etc.

Southwest airlines recently had their weather forecasting and reporting system go down which grounded their entire fleet for at least an hour. My sister was a victim of this, and I found many of the references above, but I wish I had been to this exact answer at that time. I was pretty sure the CA could just go to aviationweather.gov and see everything was VFR from SMF to SAN, modify the dispatch, and go. They didn't, but I think I would have done that if I could convince dispatch it was ok.


text:
What is an SIC type rating?
14 CFR 61.55 says:

...
(d) A person may receive a second-in-command pilot type rating for an
  aircraft after satisfactorily completing the second-in-command
  familiarization training requirements under paragraph (b) of this
  section in that type of aircraft provided the training was completed
  within the 12 calendar months before the month of application for the
  SIC pilot type rating. The person must comply with the following
  application and pilot certification procedures:
...
(6) The applicant must appear in person at a FAA Flight Standards
  District Office or to an Examiner with his or her logbook/training
  records and with the completed and signed FAA Form 8710-1.
(7) There is no practical test required for the issuance of the “SIC
  Privileges Only” pilot type rating.

What exactly is an SIC type rating used for and how can someone get a "type rating" without any kind of practical test?

It's an ICAO requirement, basically it's just official documentation showing that you've been trained and are  competent to act as SIC in a given airplane.
ICAO is an organization that standardizes regulations and procedures among the participating countries (close to 200 if I recall).  If you fly as SIC internationally you'll probably need an SIC type rating.
The rating is almost the same as a PIC type rating, except the rejected takeoff and taxi requirements (and maybe some others) don't have to be met.  It's basically a formality for any airline first officer as they will have met all the requirements just by going through the airline's training.

Looking at the excerpt that I quoted above, it says that there is no practical test required.  I think that you are describing a 121 or 135 SIC checkride in your last paragraph, not the SIC type (which has far fewer requirements than an actual type rating).

You don't have to take a formal checkride, but you have to do all the training for one and be satisfactory in it.  Checkrides are just a formality.

Actually, if you read the rest of 61.55 there is very little training that is actually required for Part 91 and it is nothing like a PIC type. Under 121 or 135 you have to complete their training program, so it is as you describe it.

A U.S. pilot can apply for a SIC type rating after receiving instruction specific to that airplane to include:

Operational procedures applicable to the powerplant, equipment,
and systems.
Performance specifications and limitations.
Normal, abnormal, and emergency operating procedures.
Flight manual.
Placards and markings

The applicant will then need to perform the required takeoff and landings as well as flying the airplane during single engine operations and become familiar with engine out procedures.
The PIC who trained the SIC applicant should sign their logbook and 8710 as the recommending instructor.  There are provisions for a person in management to make the required endorsements.
The applicant will then submit the application to the FAA either by making an appointment with their local FSDO or meeting with an examiner who has those privileges.

An SIC rating is not required for any U.S. domestic operations but may be required for operations outside the United States.  It is important that a U.S. pilot who does not plan to fly internationally is still required to meet the requirements above (also found in 61.55) they just don't need to complete the final step and get the actual "SIC type rating" on their pilot certificate.

Its a box to check to make ICAO happy if you want your operation to fly outside of the US.  For a 121 initial training program it literally is a checkbox -- you pass training and have an 8710 ready and an APD gives you the rating.
Its a formalization of knowing the airplane well enough to operate it in normal and abnormal conditions without the rigor of the PIC type (e.g. as an FO you do the oral exam with your sim partner and as Lnafziger points out, you don't cover the PTS for the SIC type like you do for a PIC type).

You don't need to cover any of the PTS for an SIC type. See my comment on his answer above.  :)


text:
Why are prop blades not shaped like household fan blades?
My two projects right now are learning to fly and 3D modeling a replacement blade for antique fan, and a question occurred to me:
Why are airplane propeller blades not shaped like household fan blades?
They're both designed to push the same fluid with some degree of efficiency, but household fan blades look a lot more like ship screws/boat propeller blades. Is it a weight issue? A visibility issue? A drag issue, and if so, how?
EDIT - This is not a duplicate of this question because the proposed answers a question about angle of attack and pitch, which are aviation jargon words that may mean nothing to novices. Pitch and angle of attack also have little to do with the front-view shape of a household fan or prop blade, which is likely the most obvious difference between the two types of blades in the eyes of someone without an aeronautical engineering degree.

Possible duplicate of How does the Angle of attack vary from the root to the tip of a propeller for a fixed pitch prop?

@RyanMortensen That seems like a completely different question to me. That question is asking about the definition of the term "angle of attack" and the mathematics around it; this question is asking about the motivation behind a design decision.

I'm not going to entertain a debate, this isn't the place. Ask on meta.  Based on feedback I've seen from far more experienced users in the past, the main determining factor seems to be based on the resultant answers of a question, not the actual question itself.

@RyanMortensen, I agree with Tanner. The question you proposed does not answer my question, and even if it did, is not phrased in a way a relative newcomer would ever have asked it. I worded my question as I did because it is exactly the question I wanted to ask.

You might want to post this on engineering as well, because the answers here don't get at the underlying issues in any coherent way. There are certainly differences in the fluid dynamics. These are characterized by differences in Reynolds number, thrust coefficient, and power coefficient between the two. And fans are often ducted to some degree. But the real difference is the cost of achieving acceptable performance.

1) Airspeed, 2) Forward motion, 3) Size constraints. Just to begin with.
Household fan blades are extremely slow, so they need more chord to push a meaningful amount of air. Aircraft propellers approach the speed of sound at their tips, and low drag is critical. All things equal, more span and less chord is more efficient. Reducing the airspeed for props has diminishing returns, because aircraft themselves move forward through the air, so a very large propeller adds drag to the aircraft however slow it is.
In household fans, size constraints are more critical than efficiency. A high-span, low-chord (narrow) blade would be more efficient in a household fan as well - indeed, you can find household fans with such blades. But they have to either spin faster for the same amount of airflow, which adds noise, or be larger in diameter. Ceiling fans, which can be larger, have longer narrower blades than desk or floor-standing fans.
Ship propellers move in an extremely dense and viscous medium, which changes things even more. The drag of moving through water is extremely high and proportional to V³ power-wise. The thrust force they produce is at most proportional to V², and can be lost to cavitation at high velocity. So their velocity has to be kept as low as possible. The drag cost of adding more chord is also relatively small in water.
On large ships, propellers are already as large as they can be made without sticking out of the water at low draft (on merchant vessels) or reducing the number of shafts that can fit (on combatants). This allows them to spin slower and lose less power to drag.

Fan blades spinning as fast as propellers would also be a safety hazard, and also probably have extra trouble with vibrations. It's just a bad idea all around :D

The key reason for this is that efficiency loss mainly occurs at the tip of a blade (where high pressure air can leak around to the low pressure side). Fat blades have more tip than narrow blades, so they're less efficient.

Submarines, which have no "sticking out of the water" problem, can have incredibly large props.

@CarlWitthoft submarines have to work to a degree on the surface.  And a screw that sticks out of the water is one someone can take a picture of... if the boat comes out of the water in drydock the screw gets shrouded.  It might be more accurate to say that submarines have very cramped hulls.

@Luaan Some household fan blades actually do spin at similar RPMs to light aircraft propellers. They just tend to be rather small fans and usually have a shroud around them to prevent any danger. And they're also not nearly as long, so the blade tips aren't moving nearly as fast for the same RPMs. Also, not nearly as much mass per blade. Computer cooling fans are a common example of this. It's not unusual for them to be in the 1,500 - 2,000 RPM range, which is around the same as the PA-28s I've flown.

@reirab: Nitpicking... My computer fan (Lenovo laptop) is currently spinning at 3061 RPM (per Conky).  And my PA-28 would cruise at ~2700 RPM.

@reirab, there's a difference between RPM and tip velocity.  Just grabbing a random cooling fan from my parts box, it's got a speed of 5500 RPM -- and a tip velocity of 11 m/s (41 km/h, 22 knots).

@Mark Yeah, that's why I mentioned the difference in tip velocity. Also, not a fan of 40 mm cooling fans, unless it's in a 1U server and that's all you can fit. :) Those things have to spin way too fast to move enough air. I much prefer 120s or, better yet, 220s or so.

@ChrisStratton The Typhoon SSBN actually does have props which break the surface while surfaced.  It's a vessel definitely designed to operate submerged as much as possible, and only surfaces to enter port.  Additionally, many merchant ships have exposed props when unloaded.

@Chromatix do you have a cite on the typhoon screws being more than at most trivially exposed when surfaced in normal trim?  It seems rather unlikely from published drawings, however it is also a design that dates from an era of being in a trailing position on screw design so potentially with fewer details to keep secret, and also an era of a more closed society more able to limit proximity of "recreational" boaters and aircraft during approach to port.

@ChrisStratton In https://upload.wikimedia.org/wikipedia/commons/9/9e/Satelite_image_of_a_Typhoon_Class_Submarine_Severodvinsk.jpg you can see plenty of the prop shrouds either side of the rudder.  In underway photos they are often masked by the considerable wake of a hull not optimised for surface travel, and the churn caused by the props themselves operating.  In some photos the shrouds also appear to be absent.

Some household fans are shaped like airplane propellers, the ones that need to move a lot of air at the highest speed. For a given motor, they have the highest efficiency, but the tip effect makes them noisy. They are best suited for industrial applications.
For inside the family home there are other considerations:

Silence. Best if we don’t hear the fan run at all.
Low air speed. We only want to feel the cooling effect of moving air, not have our hair blown out of shape.
Purchase cost. This is the one we directly see. The usage costs are hidden in the monthly electricity bill.

So for that purpose, a slow moving tip with many broad blades is best.

Let's try helicopter vs ceiling fan:  helicopter - symmetrical airfoil. Ceiling fan - flat.  Note long thin fan blades will help reduce interblade turbulence issues for a high speed industrial fan, but optimal airmoving shape that produces the LOWEST "lift" and LEAST DRAG would likely be a thin plate.

@RobertDiGiovanni ceiling fans tend towards flat blades because of their lower production cost and because they are often intended to operate in reverse with some efficiency, not because an airfoil would be bad. I have owned several standing fans with airfoil shaped blades, as well.

"usage costs are hidden in the monthly electricity bill."  Maybe about 0.5-2$ USD per month?

@chux $2 per month adds up to the purchase price of a cheap desktop fan in a year. Of course you can pay 10 times as much for a fan that does exactly the same thing, if you want to :)

I think it should be obvious that a fan blade is basically a wing travelling in a circle. So the question is what makes a wing a good wing?
In theory and in practice (wind tunnel measurements or fuel consumption measurements of real planes) a long skinny wing is significantly more efficient than a short wide wing. The most efficient wing for a given airfoil is actually one with infinite wingspan. But since there are only a finite number of electrons in the universe it is not possible to build this perfect wing. Instead engineers try to make wings/propellers/fan blades as long and as skinny as practical given the limitations of materials.
You will even see this effect in household fan blades: in situations where span is not limited, such as ceiling fans, the fan blades tend to be long and skinny instead of short and wide like ship propellers. So why short and wide? Space restriction.
When the fan must fit on a tabletop, you cannot have 1 meter long blades. But as soon as this restriction is lifted you will see designers shift to long skinny blades like what you can find on industrial floor standing or wall mounted fans.
Granted, there are floor standing fans with the exact same blades as tabletop fans but this has more to do with reusing parts and economies of scale than aerodynamics.
Now, you can in theory also have an efficient short skinny fan on a tabletop fan: just look at drone/RC airplane propellers - they're often way shorter than the fan blades of tabletop fans. But you will need to move the blades really fast to move a given amount of air. This is very noisy (have you ever seen a drone fly?). So the second consideration is noise. To reduce noise you move the blades slowly. A slow blade doesn't move much air so you increase the chord to make them wide. This results in a very inefficient fan but efficiency is not your main concern: you are designing a non-moving machine sitting on a table drawing power from a socket in the wall. Noise reduction and size matters more.

+1 As an example, I used to pass through the VLC airport in the past, and it has  massive fans on the ceiling, with long and thin blades not unlike the wings on a glider, and with a similar airfoil too. The fact that building maintenance often had them rotating backwards can be explained via Hanlon's razor. EDIT: see this question https://aviation.stackexchange.com/questions/61833/what-s-this-vortex-generator-like-thing-below-the-wing

Price is the main reason. Ceiling fans just stir the air around and flat paddles are the cheapest way to go.
More expensive fan blades can have some aerofoil shape and even winglets at the tips but this is mainly for show since these blades don't have twist.

But wouldn't a more wing-shaped, narrower blade use less material and be less expensive?

Learning to flying and modelling a fan blade, both spin in the air but the propeller tries to move the plane, and the fan tries to move the air.
How does this factor into design?  Propellers stand above and apart from fans in that they generate lift not only by deflecting air (bottom lift), but also from their motion through the air (top lift).  This is best explained by viewing the lift curve vs AOA of an airfoil from 0 to 45 degrees.  Lift will increase up to stall, then decrease, then increase again up to 45 degrees. This means you have to move LESS air to get the same lift with an airfoil up to stall.
The fan really is opposite, unless you want it to pull it self around the room.  The fan blade is designed to move air, period. Sensible design would make it compact, and thin flat blades with a wider chord would be fine, as one just looking for a cooling breeze from their product. Keeping the original style of fan on the "antique" may make it more valuable.

You have a good point, but it doesn't need to involve the dubious concepts of 'top' and 'bottom' lift. Simply put, propellers are optimised to create lift (thrust) while disturbing the air as little as possible. This favours a large diameter in the first place, to move more air with the lowest possible speed. A fan is almost the opposite: it needs a certain diameter and move the air at a decent speed. (A cooling fan, that is; requirements may be different for a ventilation fan).

"A cooling fan, that is; requirements may be different for a ventilation fan" which may be different from a jet fan!  Look at the shape of those airfoils!

@Zeus Tanner Swett published an airfoil Clift vs AOA graph that shows the "top lift" contribution at (lower drag) lower AOA as a lift maxima, which drops to a minima at around 30 degrees AOA, before rising to a maxima (with loads of drag) at 45 degrees.  This is the Rosetta Stone for many applications.  However, your point that the moving wing "disturbs as little air as possible" is an excellent way of describing it (hence less drag for a given lift!). Thanks.

@RobertDiGiovanni Friendly nitpick: consider using the singular form of minimum and maximum when appropriate.

Most of the other answers are already correct. Just one more aspect: Blade chord length.
With something that spins as fast as an airplane propeller, the blades need to be as light as possible, to minimize centrifugal loads. So it's better to give them a short profile chord length and high area loading (how much force they produce per surface area of blade), which means small chord length and turning the flow comparatively sharply, which requires a large pressure difference between the pressure and suction side of the fan blades. For a desk fan, any old plastic will be strong enough, so those blades can work with a very low surface loading (long profile chord length) and turn the flow slowly, with a small pressure difference, which is quieter.
The same applies for ship propellers: Since water is so dense, the structural loading on those blades is pretty high. If you tried to get the same thrust from a propeller with short chords (at equal radius), the blades would have trouble carrying the bending loads, and the sharper turning would also lead to cavitation [1] quicker.
[1]: cavitation happens when the pressure on the suction side of an underwater propeller falls so low that water evaporates. This effectively separates the flow, which reduces suction, so the freshly-evaporated water condenses again, water rushes back to where the steam bubble was... That can break a propeller very quickly.

Some rare designs do look more like a fan blade.  The Antonov AN-70 is a good example of this, sporting huge fan blades on its D-27 propfan.  The scimitar props do make for a rather efficient design, but they are extremely noisy which limits their usability around many commercial airports.

Image credit : By Tangopaso - Own work, CC BY-SA 3.0

Some additional discussion can be found in this question :
What are the advantages of more than 4 propeller blades?

Those props are wild.

[If you can not explain why wind turbines should be made with aerofoils, please have the sense or decency to not edit this answer.]
A fan blade pushes air.
An aerofoil is a special device.  It passes through the air, leaving a partial vacuum behind it.  The trailing part is designed so that this vacuum, instead of pulling the thing backwards, pulls it perpendicularly.  It is an ingenious device.  It does not push air.  If they wanted it to push air, they would have made it like a fan blade.  It is not like a fan blade.  It does not push air.  It is a cleverly designed device.
Actually, what happens is, not that the vacuum pulls, but that the air pressure pushes; the difference is that things [otherwise] almost always have equal air pressure on all sides.
Again… .  A wing is not lifted by laminar flow.  A wing is not lifted by vacuum.  A wing is pushed up, from below, by air pressure.  This works because of the ingenious design of a wing, that makes it so that, as it passes through the air, a (partial) vacuum is formed… and this partial vacuum functions to lift the wing, rather than dragging it backwards, because of the clever and special design of an aerofoil.
The reason that a propellor-driven aeroplane can not do more than about 400km/hr is that the air pressure can not push it any faster.
The reason that a wing at a steep angle of attack works less well is not about laminar flow, nor turbulence; it is about the fact that (a) the horizontal area being pushed upwards is smaller and (b) the surface being pushed is on an angle, which compromises the pushing upward concept.  Again, no amount of managing laminar flow and turbulence will change the above two facts.  (Note also that a wing at a steep angle of attack has (more) drag backwards (so to speak).)
Note that there is a systematic problem with propellors, being that they are passing through the air such that the clever vacuum side is moving towards {air at normal pressure}.  (I don’t know if that is a freebie or not, but it does not make much difference.)
If you understand aerofoils now, and no one ever explained this properly before, please leave a tick or a positive comment or something.  It would be a pleasant change from people looking at me funny or getting angry, and I would really appreciate it.

I don’t intend to be mean here but I don’t understand how this attempts to answer the question as asked by the OP.  The Question asks why propellers aren’t shaped the same as a house fan.

No attempt at all to answer the question that I can see, just a long & disjointed (and uninteresting) rant.

“They're both designed to push the same fluid with some degree of efficiency” is false… as I say literally about 12 times.  I am open to someone explaining to me that I do not understand aerofoils.  At this point I am still confident that I am addressing the OP’s failure to understand that aerofoils do not work by pushing air.

@Ralph J:  not meaning in the slightest to be nasty, but… do you have any account of why I might think that a wind turbine should have an aerofoil blade?

@ daleam:  In my other post, that I deleted, [from memory, offhand] I had a longer, more in-depth description of how an aerofoil works… and it made no difference.  Theoretically I could try to find out how to include a picture of airflow over an aerofoil, but 99.999% of readers would have already seen that.  Vacuum!  Would you suggest that I spend four paragraphs explaining aerofoil vacuum [serious question]?

Note that the first sentences are useless if you can express yourself in a clear way (which is preferable). Moreover, whatever your answer, you MUST answer the question. See the help center for more information.

If you can add a bit about the so called "pusher prop" (see the S-97 Raider among other aircraft) that m ight clear up your push/pull illustration a little bit.

The question is very clear.  “Why are airplane propeller blades not shaped like household fan blades?”  I believe that the difference is that a household fan blade pushes air, and an aerofoil creates a vacuum — adapted in a clever, special way.  With genuine, great respect… if none of you highly trained and experienced aeronautical engineers can explain that that is wrong, I shall continue to believe it.  Failing that, the objection must be that my answer is not about the difference between pushing air and creating a vacuum in a clever way.  If it is my wording or tone or something, say so.

…Or maybe that [the quote from the question] is open to interpretation?  Conversely, the comment about pushing fluid… to me makes it clear.

@KorvinStarmast:  Brilliant stuff.  I am guessing that the main rotors can not be accelerated relatively to each other, to twist the helicopter… suggesting/implying that the tail propellor can steer.  I am also inferring that it can be feathered backwards to enable the tilt-forward hover.  On the topic:  I would be astonished if it (the tail propellor) does not have aerofoil blades (like any other propellor-driven aeroplane).

On my fifth or tenth (?) reading of the question, I am taking it that everyone else is taking the question to be about the fact that an aerofoil propellor blade is straight, and a fan blade trails distally.  Is that right?  Is that the issue?

I probably should not say this, but it means a very great deal to me that you folk are prepared to discuss this issue, rather than just deleting the question and walking away.  Thank you all so much!!!!


text:
What happened to the Japanese aviation industry after WW2?
The Empire of Japan had a domestic aviation industry supplying the Imperial Japanese Army Air Force up until the end of WW2.
The USSR and Western Europe, which saw large parts of their production capability destroyed or diminished and their top people killed, purged, or poached, were able to build their aviation industries after the war, but Japan did not.
Why did domestic aviation production in Japan die with the war? As far as I've read, there were no mass purges or defections, and there were no concerted Operation Paperclip-style attempts to poach top talent from Japan after the war, and Japan was able to build up world-class manufacturing in many other sectors very quickly.

Japan started (continued) building aircraft not long after the war.
Kawasaki Helicopters
https://global.kawasaki.com/en/mobility/air/helicopters/index.html
Mitsubishi engines (these are in the Japanese model of the F-15)
https://www.mhi.com/group/mhiael/products
Fuji (in 1955)
https://en.wikipedia.org/wiki/Fuji_LM-1_Nikko
Mitsubishi F-1
https://en.wikipedia.org/wiki/Mitsubishi_F-1

So to some extent this is a visibility issue, but the scale of production is dwarfed by the West... I'm wondering if there were any post-War treaty prohibitions, like those placed on powered flight in Germany after WW1.

Hondajet (one of the most sold light businesses jet) is missing in your list

Yeah, that wasn't meant to be a comprehensive list. But good catch.

@sophit, Yes, technically "after WWII", but the Hondajet is super recent compared to the theme of what happened to pre-war airplane companies during post-war rebuilding of a ravaged economy...

Mitsubishi Diamond, design purchased by Hawker. And in the wiki list of military aircraft of Japan, we can find quite a few from Fuji, Kawasaki, Mitsubishi,  and Shinmaywa.

@MichaelHall: well, if you prefer you can interpret my comment as "the Japanese aviation industry is still here and actually even expanding"


text:
Does taking a long time to solo mean that I'll take a long time for training in general?
I have a fairly high (in my research - 80hrs and counting) flight hours and still no solo, with landings having been a major reason. There is definitely progress - but I am not convinced I'll solo before 100. I am 43 in decent shape and reasonably coordinated (no red flags).  In addition to being expensive and frustrating it begs a question whether any subsequent flying activities like an instrument rating would also take significantly longer (thus being more expensive and frustrating) and - more importantly - whether it is an indicator that my future piloting skills will always be mediocre.
My CFI says that "everyone hits a plateau" but I am concerned that past may be an indicator for future...
Couple notes to address comments
- I have passed my medical
- Hours were spread over 10 months and there was a 2-3 month lull (due to weather + ski season) in winter and 1 month break in summer.  Otherwise I was pretty religious about 2-3/week lessons.
- I actually changed CFI at about 50hr mark and quite like how my current CFI teaches
So while it would be nice to look at outside issues - I think the problem is more likely to lie with me than with others...  Now back to my original questions...
Any thoughts?

Not an answer, but: If it's mainly landings that you're having trouble with, and you hope to later move on to e.g. get an instrument rating, maybe your instructor would be willing to let you do one or a few lessons of simulated instrument time? That way, you could benefit from their experience in instructing students in instrument flight to see if that's something you might have an easy or hard time with when the day comes. Also, have you considered flying with a different instructor, even if for just a few flights? Sometimes getting a fresh perspective can help a lot.

That long to solo is either a significant issue with continuity (hours spread over many, many months), a significant issue with the instructor, or else a significant issue with the student -- in which case a competent instructor owes you a FAR better & more detailed explanation than "everybody hits a plateau".  My first guess: issue may be in part or mostly with your CFI.

@Aleks you may like how they teach, but that doesn't necessarily mean they're good at it.

Yes, that much time pre-solo is a huge red flag.
If you have 80 flight hours logged and no solo, unless there is some unusual extenuating factor (e.g. those 80 hours were spread over many years), something is seriously wrong. Either:

You do not have the right aptitude for flying
Your instructor is incompetent, or
Your instructor is deliberately milking you.

If the instructor hasn't already had a frank discussion with you about your progress and prospects by this point, that would indicate #2 or #3.

Second this. Try flying with a new instructor for a few lessons. One not associated with your old instructor or school. Preferably somebody with significant experience instructing and good references. Local FSDO can probably recommend a few who are good that way.

@RalphJ FSDOs are prohibited by law (or maybe it's just policy) from recommending any particular CFIs, DPEs etc. They can give you a list of all the CFIs registered in their district and their contact info but won't help you pick.

It depends on few factors.
If you had some pauses in your flying or if you are unable to keep the regular pace in your lessons, that may slow you down. Flight students tend to lose coordination fast.
I suppose that you have passed your medical? There is an eye condition that may prevent you from seeing the depth correctly. I hope that this is not the case as this usually disqualifies you to be a pilot.
I am 47 myself. Very young people tend to advance faster before first solo as they are able to learn coordination faster. During the route flying older people have more patience and general knowledge, so they advance faster. That is not the same set of skill.
In the end, you should discuss that subject with your instructor. Your hours are a bit long for the first solo, so it might be smart to discuss the root cause. This "open end" situation leads to frustration. You might lose motives and they may lose a student. It looks to me that tis is a good time for the "next steps" conversation.
In the end, I know some people that were in the same situation as you and they are great instructors now.

I would second the responses that say, "Not necessarily." It really does depend on what's holding you back, but in general, the ability to master the skills of landing are quite different from all of the knowledge/memory items required for cross country, decision-making, etc.
Also, and this may be completely irrelevant for you, but I heard about a person who struggled for so long to make a good landing her CFI was ready to give up on her - told her she just wasn't cut out to fly. Another CFI did one flight with her, got her a pillow to sit on so she could see the ground, and she soloed within another hour or two of lessons. Just to say, it could be something pretty simple.

At the risk of answering a question with a question, what about "landings" is holding you back?
2 recommendations:   First, fly the plane all the way down to the ground.  A landing doesn't just happen, you control every step.  Break the landing pattern down into parts and gain proficiency at each one.  "Landing" will be much less stressful, rather than "facing it" as one stressful event.  Second, at this point, before flying, sit down with your instructor and be perfectly clear you WANT control of the plane and DO want to get past this step so you can solo, when you feel you are ready.
This can be worked out by incrementally gaining proficiency in entering downwind, lowering to approach speed, judging glide angle, controlling flaps and power, stable final approach, rounding out, flaring and landing.
Try to focus on your weakest point, and work to improve it.
Your instructor should not "jump in" unless you need help.  Do lots of touch and goes and work to gain confidence in each step.
Not everyone solos after 20 hours.

While this may be good advice, I feel it doesn't answer the question whether a long time until first solo is an indicator for total required training time.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

I don't disagree with that statement, but I don't think this question is entirely unanswerable. Difficulties to acquire a certain skill (e.g. perception of visual cues & motor feedback skills) in one phase can be indicative of difficulties to acquire similar skills in a next phase. However, if the next phase requires totally different skills, then there may be no difficulties.

@DeltaLima  well, especially since the training was broken up and not consistent, very difficult to draw a conclusion other than to get a good instructor and focus on getting past step 1.


text:
What’s the difference between “approved” and “legal” procedure?
I occasionally see these two words used in opposition; for instance [emphasis mine]:

In the United States, aircraft usually join the pattern at a 45° angle to the downwind leg and abeam midfield. Although aircraft may legally join the pattern at any point, the AIM clearly states that the only approved pattern entry is the 45[° angle entry].1

In the referenced circular AC 90-66B CHG 1, the words “approved” and “legal” don’t appear except in other contexts.
So,

Is this just imprecise layman terminology in use on Wikipedia?
Or do the two terms have defined meaning somewhere?

My guess is that a pilot-in-command can deviate from “approved” procedure to other “legal” procedure whenever deemed necessary by exercising PIC authority, but deviation from “legal” procedure would be allowable only in emergency (“mayday”) or urgency/distress (“pan-pan”) situations? Or perhaps also if controllers direct or authorize deviation?2
I’m curious primarily about flying under FAA and ICAO regulations, but if this terminology differs in other jurisdictions, I’d be interested in that as well.

1 From English Wikipedia, “Aircraft traffic pattern”, retrieved 16 Aug 2019 20:30Z
2 Wording in the circular §8.2.1:

The FAA does not regulate traffic pattern entry, only traffic pattern flow.

gives a clue in specifying that the 45° entry is not regulated.

The AIM is not regulatory, just best practices approved by the FAA. You should do what the AIM says, but as PIC you are free to ignore it.
The FARs are regulatory, so as long as you don't violate them, what you're doing is legal. You must do what the FARs say (unless you have an emergency, of course).
In the case of pattern entry, other pilots are expecting you to do what the AIM says (45 to downwind), so doing something different is probably unsafe--especially if you aren't in radio contact. But if there is nobody else in the pattern, that reason doesn't apply and it is safe to enter however you wish (straight in, crosswind, etc.). It is up to you as PIC to make this decision.
The same logic is true generally: if you understand why the AIM says to do (or not do) something, you probably understand when it's okay to ignore it.

That makes sense (particularly in light of what I added under footnote 2 I edited at more or less the same moment you posted this answer). But what does this mean with reference to my guess regarding PIC authority? If you could add that, I’ll accept this answer. Thanks!